Field of the Invention
The present invention relates a stepping motor, a timepiece movement, a timepiece, and a manufacturing method of a stepping motor.
Background Art
In the related art, an analog electronic timepiece has been used in which a motor drive device rotatably drives an indicating hand such as an hour hand and a minute hand. The motor drive device has a stepping motor for rotatably driving the indicating hand and driving means for rotatably driving the stepping motor.
The stepping motor has a stator which has a positioning portion (inner notch) for determining a stop position of a rotor accommodating through-hole and a rotor, the rotor which is rotatably arranged inside the rotor accommodating through-hole, a magnetic core which comes into contact with the stator, and a coil which is wound around the magnetic core.
A configuration is adopted in which drive pulses having different polarities are alternately supplied from a drive circuit to the coil so as to cause the stator to alternately generate magnetic leakage fluxes having different polarities. In this manner, the stepping motor, that is, the rotor is rotated by every angle of 180 degrees in a predetermined one direction (forward direction), and the rotor is stopped at a position corresponding to the positioning portion.
In general, in order to easily obtain the magnetic leakage flux for rotating the rotor, an integrated stator is used which has a narrow portion whose width is narrowed at two locations (apart from each other by an angle of 180 degrees) around the rotor accommodating through-hole formed to arrange the rotor so that the narrowed portion is easily saturated with magnetic flux.
As a technique used in order to easily obtain the magnetic leakage flux for driving the rotor, a so-called dually integrated stator is known. In the dually integrated stator, the stator is first cut and divided into two at two locations (apart from each other by an angle of 180 degrees) around the rotor accommodating through-hole so as to minimize a cross-sectional area of a magnetic path. The cut portion is welded and joined after a slit material formed of a low magnetic permeability material or a non-magnetic material is inserted into the cut portion so as to decrease magnetic permeability of the narrow portion (refer to JP-B-5-56109).
However, the technique in the related art has residual tasks in the following points.
In a case of the above-described integrated stator in which the narrow portion is formed at two locations around the rotor accommodating through-hole, a principle of driving the rotor is that the narrow portion is first saturated with the magnetic flux and the stator is magnetically divided into two magnetic pole pieces. Thereafter, the magnetic leakage flux flows to the rotor so as to rotate the rotor. That is, the magnetic flux generated from the coil when a current is supplied is consumed by the narrow portion (power is consumed in order to saturate the narrow portion with the magnetic flux), thereby causing a problem of magnetic flux loss in the narrow portion.
Since the narrow portion is present, the magnetic flux generated from the rotor itself is consumed by the narrow portion. Thus, it becomes difficult to obtain a peak of magnetic potential, thereby degrading a retaining force for magnetically stopping and retaining the rotor. As a result, there is a possibility that an operation for stopping the rotor at a position corresponding to the positioning portion may be unstably performed, or that the rotor may be rotated (stepped-out) beyond an angle of 180 degrees.
According to the technique disclosed in JP-B-5-56109, the stator is divided into two pieces by means of machining, and thereafter the two pieces are joined to each other by means of welding. Thus, mechanical stress, distortion during the welding process, or misalignment of members is likely to occur. For this reason, there is a problem of an error occurring in a distance between the rotor and the stator. Consequently, there is a problem in that disadvantages such as a misaligned stop position of the rotor and inaccurate rotation are likely to occur.
If an outer shape of the stator is distorted, the stator is no longer flat. Thus, a contact area between the coil and the stator is likely to decrease, or mutual positions between the rotor and the stator are likely to be misaligned with each other. As a result, there is a possibility of poor magnetic efficiency or damage to the stator during an assembly process, thereby leading to poor quality products.
Therefore, the present invention is made in view of these circumstances, and an object thereof is to provide a stepping motor, a timepiece movement, a timepiece, and a manufacturing method of a stepping motor, in which power consumption can be reduced (power saving) and which can improve stability of a rotor rotatably driven by using a high retaining force.